The performance characteristics of a Complimentary Metallic Oxide Semiconductor ("CMOS") device are maximized when the device operates at a certain supply voltage (the "optimal supply voltage" V.sub.OPT). Although the value of the optimal supply voltage for a CMOS device is specified when the device is designed (the "design voltage"), variations and imperfections in the manufacturing process cause the optimal supply voltage to vary from device to device. Moreover, when the CMOS device is placed in operation, the voltage supplied to the device is rarely equal to the optimal supply voltage, or even the design voltage. In fact, the supply voltage will likely vary over time as operating conditions change.
As a result of these variations, CMOS devices are designed to operate within a specified voltage range, such as plus or minus five to ten percent, around the design voltage. The bottom end of the specified voltage range is limited by the threshold voltage of the CMOS device, which is the minimum voltage required to operate the device. When the voltage supplied to the CMOS device is at or near the threshold level, the device will operate, but at a much slower speed. Conversely, the top end of the specified voltage range is limited by the reliability constraints of the CMOS device. When the voltage supplied to the CMOS device is at or near its maximum operating level, the device will operate at maximum speed, but its power dissipation will be excessive. Accordingly, the goal of any voltage regulation circuit should be to maintain the supply voltage to the CMOS device at or near the optimal supply voltage for the device. Most voltage regulators, however, maintain the supply voltage at or near the design voltage; not the optimal supply voltage.
As discussed above, the manufacture of integrated circuit devices is not 100% repetitive. That is, the geometry of each device varies due to imperfections and variations in the manufacturing process, which in turn affects the performance characteristics of the device. Although, these devices are designed to operate within a specified voltage range, these imperfections and variations may cause some of the devices to operate too slowly, dissipate too much power, or not operate at all, at the minimum or maximum specified voltage, thus making those devices unusable and thereby decreasing the production yield. Furthermore, the production yield for a given device decreases as its complexity increases.
One approach taken by some designers and manufacturers to increase production yield is to provide jumpers or programmable connections on the device to alter its performance so that it operates within the specified voltage range. This method, however, only provides a "coarse" adjustment and does not assure that the device will operate at its optimal level for a given voltage, and will not track the performance of the device, especially over temperature variations.
These problems are multiplied as circuit designers and manufacturers continue to decrease the overall geometry of CMOS devices because the design voltages and associated operating ranges are also decreased. For example, to achieve a certain gate length, such as 0.25 microns, the supply voltage cannot exceed 3.3 volts. If the gate length is decreased to 0.18 microns, the supply voltage cannot exceed 2.0 volts. This smaller voltage range will necessarily further decrease the production yield. Moreover, systems designers employing CMOS technologies often limit the devices they use based on a range of supply voltages. As the dimensions and supply voltages of these devices decrease, the devices themselves may become unattractive to designers who are limited in device selection based on system requirements.
Accordingly, it is desirable to have a circuit and method for intelligently regulating a supply voltage to a served device in which the supply voltage is regulated according to one or more performance parameters of the served device.